The invention addresses a novel production method for digital X-ray detectors, such as are used inter alia in medical diagnostics. Detectors based on amorphous silicon (for indirect conversion) and amorphous selenium (for direct conversion) represent the current state of the art. The principles of direct conversion (left) and indirect conversion (right) are illustrated in FIG. 1. With direct conversion I, an X-ray quantum 1 excites a particle 2, electron/hole pairs 2a, 2b being generated, which pairs then migrate to the electrodes 4 (anode or cathode, pixel electrodes, for example), where they are detected. With indirect conversion II, the X-ray quantum 1 excites the particle 2, which again emits radiation 2′ with lower energy (for example, visible light, UV or IR-radiation), which is then detected by means of a photodetector 3 (e.g. a photodiode).
Indirect X-ray conversion therefore contains, for example, the combination of a scintillator layer (for example, Gd2O2S or CsI with different doping agents such as terbium, thallium, europium, etc.; layer thicknesses typically 0.1-1 mm) and a photodetector (preferably a photodiode). The emission wavelength of the scintillator light resulting from X-ray conversion overlaps in this case with the spectral sensitivity of the photodetector.
In the case of direct X-ray conversion, the X-rays are once again, for example, directly converted into electron/hole pairs and these are read out electronically (amorphous Se, for example). Direct X-ray conversion in selenium is usually carried out with layers that are up to 1 mm thick, which in the kV range are blocked in the reverse-biased direction. Whilst indirectly-converting detectors in particular have prevailed due to their being simple and economical to produce, direct converters have a clearly improved resolution capacity.
The production of detectors usually includes the insertion of the inorganic absorber materials, such as quantum dots or typical scintillator materials, into an organic matrix. Organic semiconductors can easily be applied from the liquid phase to large surfaces and through the direct inclusion of the inorganic scintillator particles, optical cross-talk can be clearly minimized.
Unlike inorganic semiconductors, organic semiconductors have a lower conductivity. This limited conductivity becomes problematic when, as is the case with X-ray absorption, for example, very thick layers are required to achieve an adequate sensitivity. First, this reduces the efficiency of the photodiode since charge-carrier extraction is impeded. Second, the speed of the photodiode decreases, which very much restricts any use for medical technology devices. In addition, the added X-ray absorbing particles, which are not conductive, have a negative effect on the charge-carrying channel and extend it.
Organic semiconductors are predominantly applied from the liquid phase or by vapor deposition in a vacuum. All the methods known hitherto for the admixture of inorganic absorber materials involve processing from the liquid phase:
U.S. Pat. No. 6,483,099 B1 discloses the option for X-ray detection with a scintillator layer on an OPD (organic photodiode). Further designs are X-ray detection by the admixture of scintillators into an OPD, using a scintillator as a substrate or as part of the electrode. No information is given as to how a scintillator can be incorporated homogeneously into a thick OPD layer or how a hybrid diode that is 100 μm thick, for example, can be manufactured.
DE 101 37 012 A1 discloses a design for a light-sensitive and polymeric absorber layer with embedded scintillator particles. The conductivity of the polymer layer increases as a result of absorption of light from the scintillator. The average distance between the scintillator particles in the layer corresponds to the mean free path of the photons from the scintillator in the polymer.
DE 10 2008 029 782 A1 describes an X-ray detector based on quantum dots which are admixed into the organic semiconductor matrix. In this concept, the quantum dots are dispersed into the organic semiconductor solution. Here ligands, such as oleic acid or suchlike are used, which may affect the electrical properties of the organic semiconductor.
DE 10 2010 043 749 A1 relates to an X-ray detector based on the above concept, wherein scintillators are either dispersed directly into the organic semiconductor solution or sprayed on in a “co-spray process” at the same time as the organic semiconductor material.